How to Balance Chemical Equations
Atoms cannot be created or destroyed in a chemical reaction. Balancing equations is simply making both sides of the arrow count the same.
Why Equations Must Balance
The law of conservation of mass states that in any chemical reaction the total mass of the reactants equals the total mass of the products. At the atomic level, this means every atom present before the reaction must appear somewhere in the products — no atom disappears and none is conjured from nothing.
An unbalanced equation is not wrong about which substances react; it is incomplete because it does not show the correct proportions. A balanced equation tells you how many molecules of each substance participate.
You balance an equation by adjusting coefficients (the numbers in front of each formula). You are never allowed to change the subscripts inside a formula — that would change the substance itself.
The Step-by-Step Method
Step 1 — Write the unbalanced equation
Start with the correct chemical formulas for all reactants and products. For example, hydrogen gas reacting with oxygen gas to produce water:
Step 2 — Tally atoms on each side
Count every type of atom on the left (reactants) and on the right (products):
- Left side: 2 H, 2 O
- Right side: 2 H, 1 O
Oxygen is out of balance (2 vs 1).
Step 3 — Add coefficients to equalize
Place a coefficient of 2 in front of H₂O to double the oxygen on the right:
Now recount: Left — 2 H, 2 O. Right — 4 H, 2 O. Oxygen is balanced, but hydrogen is not (2 vs 4).
Fix hydrogen by placing a coefficient of 2 in front of H₂:
Final tally: Left — 4 H, 2 O. Right — 4 H, 2 O. Balanced.
A More Complex Example: Combustion of Methane
Methane (CH₄) burns in oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O):
Tally (unbalanced): C: 1/1, H: 4/2, O: 2/3.
A useful strategy: balance carbon and hydrogen first, then fix oxygen last.
- Carbon is already balanced (1 each side).
- Put 2 in front of H₂O to balance 4 hydrogen atoms:
CH₄ + O₂ → CO₂ + 2 H₂O - Recount oxygen on the right: 2 (from CO₂) + 2 (from 2 H₂O) = 4. Put 2 in front of O₂:
CH₄ + 2 O₂ → CO₂ + 2 H₂O - Verify: C 1/1, H 4/4, O 4/4. Done.
Practical Tips
- Start with the most complex molecule. The compound with the most different elements is usually the best place to begin.
- Leave pure elements (O₂, H₂, Fe) for last. Their coefficient can absorb whatever is left over.
- Use fractions temporarily. If you need an odd number of a diatomic molecule, use ½ as a placeholder, then multiply the entire equation by 2 to clear it.
- Check every element. After placing your final coefficients, count every type of atom once more to confirm nothing was missed.
Students sometimes change a subscript (e.g. writing H₃ instead of H₂) to make the numbers work. This creates a different molecule that does not exist in the reaction. Only ever change coefficients.
Practice Equation
Try balancing this on your own before reading the answer:
Answer: 4 Al + 3 O₂ → 2 Al₂O₃ (Al: 4/4, O: 6/6).
Summary
Balancing chemical equations follows three rules: write correct formulas, count atoms on each side, and adjust coefficients — never subscripts — until both sides are equal. Work from the most complex molecule outward, and save pure elements for last. Once you internalize conservation of mass as the guiding idea, the mechanics become routine.